The person who arrived on Monday, who didn't like spaghetti, arrived earlier than the person with the blue shirt.

I made this pretty obvious by including the "who didn't like spaghetti" phrase, but in the book, this extra clue might be harder to spot.

Happened to have my copy of Vol. 1 handy, so had a gander at the clues. Complex clues and potentially relevant asides are bolded.

1. Old Siam doesn't have the most open seats available and doesn't show on Wednesday. Wednesday's show has two fewer open seats compared to Music Ally.
2. Of Aunt Agony and Saturday's show, one has four open seats available and the other plays at the Gibson Theater.
3. Sunday's show (which isn't at the Roxie Theater) has the most open seats available of all five shows.
4. The Gibson Theater shows its performances either on Thursdays or Saturdays.
5. Of Komedie Kiev and the show performing at the Burdette Theater, one shows on Friday and the other is completely sold out - not a single ticket available.
6. The Porter Theater never has performances on Thursdays.
7. Aunt Agony isn't playing at either the Roxie or Burdette Theaters.

Now I have a copy of Volume 1, and there are only 7 clues just like Ace showed us. I couldn't find any other hidden clues in the backstory paragraph either.

So there are lots of solutions.

I will change one of the clues and add an eighth clue to get a unique solution:

1. Old Siam doesn't have the most open seats available and doesn't show on Wednesday. Wednesday's show has two fewer open seats compared to Music Ally.
2. Of Aunt Agony and Saturday's show, one has four open seats available and the other plays at the Gibson Theater.
3. Sunday's show (which isn't at the Roxie Theater) has the most open seats available of all five shows.
4. The Gibson Theater shows its performances either on Thursdays or Saturdays.
5. Of Komedie Kiev and the show performing at the Burdette Theater (which has fewer open seats than the Grand Theater), one shows on Friday and the other is completely sold out - not a single ticket available.
6. The Porter Theater never has performances on Thursdays.
7. Aunt Agony isn't playing at either the Roxie or Burdette Theaters. 8. The five shows are Music Ally, Old Siam, the one showing at the Roxie, the one showing on Saturday, and the one with four open seats.

I tried to come up with clue modifications that would preserve the difficulty level of the puzzle. It looked to me like at least one aligned pseudo-pair was involved.

If anyone is still working on this one, I hope this helps, and I'll take a look at those other puzzles that seemed to be missing a clue now that I have the book.